The invention relates to a process for producing α-, β-unsaturated carboxylic acid esters.
U.S. Pat. No. 4,529,816 describes a conventional acetone cyanohydrin (“ACH”) process for the production of methyl methacrylate (“MMA”) from acetone cyanohydrin. In this process, ACH is hydrolyzed by sulfuric acid to produce α-hydroxyisobutyramide (“HIBAM”) and α-sulfatoisobutyramide (“SIBAM”). Next, the HIBAM and SIBAM are thermally converted to 2-methacrylamide (“MAM”) and a small amount of methacrylic acid (“MAA”). The MAM is esterified with methanol to produce the desired MMA product, while residual HIBAM is esterified to methyl α-hydroxyisobutyrate (“α-MOB”). The esterification product stream is a mixed product which is subjected to separation and purification steps to isolate the MMA product from the other compounds. Typically, a purified MMA product stream is produced, along with a purification residue comprising other compounds including, but not limited to, α-MOB and methyl β-methoxyisobutyrate (β-MEMOB). The recovery and conversion of one or more of these other compounds to produce additional MMA product has been the subject of various research and development efforts having varying degrees of success and practical utility. In particular, U.S. Pat. No. 4,529,816 describes an improvement wherein the α-MOB is isolated and recycled to the process between the thermal conversion and esterification steps.
U.S. Pat. No. 5,393,918 describes a process similar to that of U.S. Pat. No. 4,529,816, but the thermal conversion step is eliminated. Instead, the hydrolysis product is subjected directly to esterification with methanol to produce an esterification product which includes the desired MMA product, as well as α-MOB and β-MEMOB. In the process described in U.S. Pat. No. 5,393,918, the esterification (“crude MMA”) product is subjected to distillation to recover the product MMA and produces a liquid residue stream comprising α-MOB and β-MEMOB. The α-MOB and β-MEMOB are separated from the residue stream, typically by fractional distillation. The recovered α-MOB and β-MEMOB are subjected to vapor phase catalytic dehydration, using a crystalline aluminosilicate, to produce a recycle mixture comprising MMA, MAA, methanol and water, which mixture is recycled to the process between the hydrolysis and esterification steps, or between the esterification and separation steps. The crystalline aluminosilicate catalyst, may be promoted with an alkali metal or a platinum group element.
Catalysts containing Cs and silica gels have been explored for a number of reactions, including dehydrations, aldol condensations and Michael additions. EP 0 545 318 and U.S. Pat. Nos. 4,841,060 and 5,625,076, for example, disclose catalysts containing silicon and at least one element selected form the group consisting of alkali metals and alkaline earth metals for intramolecular dehydrations, such as mercaptoalkanols to alkylene sulfides, alkanolamines to cyclic amines, N-(2-hydroxyethyl)-2-pyrrolidone to N-vinyl-2-pyrrolidone, and tertiary N-(2-hydroxyalkyl) carboxylic acid amide to tertiary N-alkenyl carboxylic acid amide. The substrates and reactions involved in these processes, however, differ chemically from dehydration and demethanolation of α-MOB and β-MEMOB, respectively, to MMA. U.S. 2002/0055650 discloses a process for preparing methacrylates by reacting α-hydroxy-isobutyric acid or its esters with an alcoholic compound in the presence of a catalyst comprising cesium on a support.
U.S. Pat. No. 4,801,571 discloses a process for the production of α, β-ethylenically unsaturated monocarboxylic acids, using a supported SiO2—SnO2 mixed oxide catalyst containing Cs, by aldol condensation of formaldehyde with, for example, propionic acid. Silica supports modified with Bi, Ti, and Ge were investigated. Catalysts prepared with these materials generally performed poorly or had unacceptably short lifetimes.
U.S. Pat. No. 6,544,924 discloses a catalytic process for the production of ethylenically unsaturated acids or esters, such as methyl methacrylate, by aldol condensation of formaldehyde with methyl propionate. The catalyst contains a porous high surface area silica, an alkali metal and at least one modifier element selected from B, Mg, Zr, Al, Mg, and Hf.
It would be desirable to have an improved process for the conversion of α-MOB and β-MEMOB to MMA.